I assume you don't actually mean that Fitbit said you only burned 693 calories. That's a very low number for most of a day unless you're a small child. I assume that what you mean is that 693 is your "Fitbit Calorie Adjustment" in MFP.

Fitbit shouldn't interpret a high heart rate as "exercise" unless you're also moving around. However, if you're taking steps, and your heart rate is high, then it might. Look at your Fitbit Dashboard and/or the Activities Page of your log (this is assuming you're using the website, but look for the appropriate places for similar information on the mobile app) and see if you can correlate your high heart rate with when you were taking a lot of steps.

If you just got your Fitbit, then it takes a few days (or more) to "learn" your resting heart rate and the like, so until it has had time to get to know your typical heart rate and activity pattern it could be off by quite a bit.

I have the same issue! Something like 5000 steps and a 600 calorie adjustmenst? Up from 1700!! Something's not working here. I've had my fitbit for almost a year.

Interestingly though I also had bad anxiety yesterday and today.